The present invention relates to blowmolding apparatus in general, and more particularly to improvements and blowmolding apparatus wherein the conversion of tubular or strip-shaped parisons into bottles or other types of hollow shaped articles takes place in several stages.
It is already known to convert tubular or strip-shaped parisons of plasticized synthetic thermoplastic material into hollow shaped articles in two stages the first of which includes converting the parison into a hollow blank or preform and the second of which includes converting the blank into a finished hollow article. As a rule, the conversion of parisons into preforms and the conversion of preforms into shaped articles involves blowing a gaseous medium into the parisons and preforms by resorting to blowing mandrels or blowpipes which can cooperate with the respective molds to calibrate the adjacent portions of the preforms and hollow articles. A preform may but need not closely resemble the finished article.
The conversion of preforms into finished hollow articles can be performed by resorting to so-called biaxial stretching of the material of the preforms. This procedure involves maintaining the temperature of the preform within a narrow range for optimum orientation of molecules of the plasticized material and subjecting the preform to a predetermined reproducible stretching or expanding action. The procedure is known as injection molding if the mold for conversion of parisons into preforms receives a parison which is injected into the mold cavity.
U.S. Pat. No. 2,715,751 to Weber discloses an apparatus with several finishing molds which travel continuously or intermittently along an endless path and each of which can be opened and closed independently of the others. The space requirements and such apparatus are excessive and the initial and maintenance cost is very high.
German printed publication No. 1,084,908 discloses a modified apparatus with two molds which open and close simultaneously. A drawback of such apparatus is that the opening of that mold wherein the treatment of plasticized material is already completed cannot precede the opening of the other mold wherein the treatment requires a longer interval of time. As a rule, the mold which is used to convert preforms into finished hollow articles must remain closed for an interval of time which is longer than the minimum interval necessary for maintaining the mold for the making of preforms in closed position. Moreover, the just described apparatus is used for conversion of injection molded parisons rather than for conversion of extruded parisons. Therefore, certain problems which arise in connection with the treatment of extruded parisons are non-existent in the just described conventional apparatus. Such problems include those which are attributable to the fact that the nozzle of an extruder continues to supply plasticized material during each stage of operation of an extrusion blowmolding apparatus, i.e., also during that interval when the mold which accepts successive extruded parisons is closed and is still located at the extruding station. If the mold for the making of preforms opens and closes simultaneously with opening and closing of the mold for the making of finished hollow articles, the two molds can be moved as a unit only after the second mold is free to move, i.e., subsequent to withdrawal of a blowing mandrel from the mold for finished articles. It has been found that such delayed transport of the mold for conversion of parisons into preforms from the extruding station can affect the appearance and/or quality of the extruded material.